It is a disease  whcih is caused by a congenital disorder which arise from defect in a chromosome  causing intellectual impairment and physical abnormalities including short stature and a broad facial profile. It arises from a defect involving chromosome 21, usually an extra copy (trisomy-21). ARe few of its symptoms 
is this hereditary or not?
Down syndrome is hereditary in approximately 1% of all instances. In the other 99% of cases Down syndrome is completely random and the only known factor that increases the risk is the age of the mother (over 35).

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Down syndrome is genetic disorder. In this type of disease, there is a extra copy of a chromosome.
The disease affects the:
i) The person does not undergo physical growth,
ii) Facial features are quite different from normal features,
iii) intellectually disabled (which means the IQ of the certain person is equivalent to that of a small child of age ranging from 8-10)
iv) Their immune system is not well up,
v) They have a higher risk of health problems(hearing&vision disorders are main).